Flip-flop renal enhancement describes when an infarcted renal parenchyma alternates between hypoattentuation during the arterial phase, and hyperattenuation on the delayed phase of a contrast enhanced CT 1,2.
Flip-flop renal enhancement should not be confused with the flip-flop effect seen on MRI from bone marrow serous atrophy, or the flip-flop appearance of the liver on contrast-enhanced CT due to Budd-Chiari syndrome.
Renal infarction typically presents with one or more wedge-shaped defects within the renal parenchyma, affecting the medulla, cortex and capsular surface 2,3. Occasionally, the entire kidney may fail to enhance when the main renal artery is occluded 2.
Flip-flop renal enhancement is thought to occur when ischemic tissue develops capillary damage and increased permeability, thereby allowing contrast extravasation into the extracellular space 2.
Flip-flop renal enhancement can be seen in up to 70-75% of cases of renal infarction and may be present for several hours post contrast injection 2.
In flip-flop renal enhancement, these corticomedullary defects appear hypodense on early contrast enhanced CT, before becoming hyperdense on delayed imaging 1,2. There may also be associated fat stranding and thickening of the surrounding perirenal fascia 3.
Flip-flop renal enhancement can help differentiate renal infarct from other pathologies, most notably pyelonephritis, which also causes wedge-shaped regions of reduced enhancement during early contrast phases 1. The cortical rim sign is another useful tool, and is observed in up to 50% of cases of renal infarcts 4.
- 1. Leto C, Giambelluca D, Bruno A, Midiri M, Salvaggio G. "Flip-flop enhancement" in renal infarction. (2019) Abdominal radiology (New York). 44 (4): 1625-1626. doi:10.1007/s00261-018-1843-2 - Pubmed
- 2. Suzer O, Shirkhoda A, Jafri SZ, Madrazo BL, Bis KG, Mastromatteo JF. CT features of renal infarction. (2002) European journal of radiology. 44 (1): 59-64. doi:10.1016/s0720-048x(01)00476-4 - Pubmed
- 3. Urban BA, Fishman EK. Tailored helical CT evaluation of acute abdomen. (2000) Radiographics : a review publication of the Radiological Society of North America, Inc. 20 (3): 725-49. doi:10.1148/radiographics.20.3.g00ma12725 - Pubmed
- 4. Regine G, Stasolla A, Miele V. Multidetector computed tomography of the renal arteries in vascular emergencies. (2007) European journal of radiology. 64 (1): 83-91. doi:10.1016/j.ejrad.2007.06.007 - Pubmed